A transmission apparatus used in a radio communication system typically includes a power amplifier for amplifying a transmission signal. In a case where power of an input signal is relatively small, the power amplifier applies linear amplification to the input signal. Accordingly, linearity of input power and output power is maintained. In a case where power of the input signal is relatively great, however, the power reaches a saturation region of an input/output characteristic and causes linearity of input power and output power to be impaired. As a result, operating a power amplifier in a saturation region with high efficiency typically causes nonlinear distortion in a transmission signal amplified by the power amplifier. Nonlinear distortion can be a factor of an increase in an adjacent channel leakage ratio (ACLR), and thus, it is desirable to reduce nonlinear distortion.
To cope with this, there is a case where a transmission apparatus employs a distortion compensation technique using a predistortion method. In this method, nonlinear distortion is compensated by previously adding, to a transmission signal, distortion having inverse characteristics of nonlinear distortion generated in a power amplifier. The distortion previously added to the transmission signal in the predistortion method is also referred to as a distortion compensation coefficient, or the like. For example, a distortion compensation coefficient corresponding to a power level of the transmission signal is read from a look-up table and multiplied with the transmission signal. Thereafter, the transmission signal multiplied with the distortion compensation coefficient is amplified by the power amplifier. With this process, the distortion caused by the distortion compensation coefficient is cancelled by nonlinear distortion generated by the power amplifier, reducing the distortion in the transmission signal.
An output of the power amplifier is fed back and used for updating the distortion compensation coefficient stored in the look-up table. Specifically, the distortion compensation coefficient is updated so as to reduce an error between a feedback signal from the power amplifier and an original transmission signal before being multiplied with the distortion compensation coefficient, leading to improvement in distortion compensation accuracy. This attempt to improve distortion compensation accuracy includes consideration of performing distortion compensation using a distortion compensation coefficient for coping with a memory effect that corresponds to nonlinearity depending upon an effect of a transmission signal input to the power amplifier in the past.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2012-90158
Meanwhile, distortions generated in a transmission signal include not only nonlinear distortion generated in a power amplifier but also burst distortion, namely, distortion generated in a burst signal transmitted in time division duplex (TDD) communication, or the like. Burst distortion is distortion that causes power to increase when the burst signal rises and causes power to decrease when time elapses. Specifically, at the time of starting transmission of the burst signal, the power amplifier starts operation and warms up rapidly. This causes power of the burst signal to increase remarkably for about 50 nanoseconds (ns) from start of transmission. When several tens to several hundred milliseconds (ms) have elapsed after start of transmission, the entire power amplifier warms up. This causes the gain to gradually decrease and thus, causes the power of the burst signal to gradually decrease. Such a phenomenon in which a signal power changes corresponding to elapsed time is generally known as a droop phenomenon.
Unfortunately, however, it is difficult to compensate this type of burst distortion by the predistortion method, leading to a problem of limited improvement in distortion compensation performance of the transmission apparatus. Specifically, distortion compensation using the predistortion method, as described above, feeds back an output of the power amplifier thereby updating the distortion compensation coefficient. With such an updating procedure of the distortion compensation coefficient, however, it is difficult to properly update the distortion compensation coefficient against the burst distortion. As a result, the distortion compensation using the predistortion method would not sufficiently compensate burst distortion, possibly leading to distortion persisting in the burst signal amplified by the power amplifier.
Hereinafter, this issue will be described more specifically with reference to FIG. 10. FIG. 10 is a diagram illustrating a specific example of a time waveform of a burst signal. As illustrated in FIG. 10, a burst signal 10 and a burst signal 20 are intermittently generated with a predetermined time interval. At a rising portion of each of the burst signal 10 and the burst signal 20, power is temporarily increased by burst distortion. The power gradually decreases as time elapses. In a case where the predistortion method is applied to this type of burst distortion, update of a distortion compensation coefficient is executed at a segment where each of the burst signals 10 and 20 is generated. Specifically, at an ending portion 11 of the burst signal 10, for example, update of the distortion compensation coefficient is executed so as to compensate burst distortion at the ending portion 11. Therefore, at the time of completion of transmission of the burst signal 10, a distortion compensation coefficient to be stored in a look-up table has become a coefficient that corresponds to the burst distortion at the ending portion 11.
When transmission of the burst signal 20 is started, burst distortion at a head portion 21 is compensated by a distortion compensation coefficient. At this time, even though burst distortion has remarkably been increased at the head portion 21, the distortion compensation coefficient stored in the look-up table corresponds to the burst distortion at the ending portion 11 of the burst signal 10. This causes a failure in sufficiently compensating the burst distortion of the head portion 21 of the burst signal 20 and causes burst distortion of the burst signal 20 to persist. This means it is difficult to sufficiently compensate burst distortion by the predistortion method immediately after starting transmission of the burst signal 20.
Meanwhile, other than a technique to read a distortion compensation coefficient from a look-up table to perform distortion compensation, the predistortion method has another technique, for example, to previously add distortion to a transmission signal using a polynomial. Also with this type of technique using a polynomial, a coefficient of the polynomial is updated by feedback of an output of the power amplifier, and thus, it is still difficult to properly compensate the burst distortion.